/**
 * Extends layout with map $extension.
 * @param  {Map}    $extension              Map with additional layout settings.
 * @param  {String} $layout-key: 'default'  Key of layout to be retrieved and returned after extension.
 */
@mixin layout-extend($with-layout) {
  @if (type-of($with-layout) == map) {
    $LAYOUT: layout-extend($with-layout) !global;
  }
  @else {
    @error 'Expected $with-layout to be of type `map`, was `#{type-of($with-layout)}`.';
  }
}